Other IDE's like Visual Studio or Intellij IDEA allow to define custom text for folded regions.

There are a lot of issues that could be solved with this feature: microsoft/vscode#70794 , microsoft/vscode#3352 , #827 and other.

The proposed change is to add collapsedText property to FoldingRange interface:

interface FoldingRange {
	startLine: uinteger
	startCharacter?: uinteger
	endLine: uinteger;
	endCharacter?: uinteger;
	kind?: string;

        collapsedText?: string;
}

The problem is: what should clients that don't support char-ranges folding (lineFoldingOnly === true) do? I've described the problem in microsoft/vscode#70794
Possible options are:

1. Clients must replace the whole first line too.
2. Clients must show the collapsedText at the end of the first line.
3. Clients are free to decide how they show it.
4. Add a property to the FoldingRange to specify the behavior.